Treat calls to CanvasRenderingContext2D.clearRect as operations that clear the canvas

Source/platform/graphics/RecordingImageBufferSurface.cpp

Index: Source/platform/graphics/RecordingImageBufferSurface.cpp
diff --git a/Source/platform/graphics/RecordingImageBufferSurface.cpp b/Source/platform/graphics/RecordingImageBufferSurface.cpp
index 7edbb5de40da972b25bf3369a8ad57f127d7ecdf..f3326eeb433aece11a133590778ace94de285f0b 100644
--- a/Source/platform/graphics/RecordingImageBufferSurface.cpp
+++ b/Source/platform/graphics/RecordingImageBufferSurface.cpp
@@ -37,6 +37,7 @@ void RecordingImageBufferSurface::initializeCurrentFrame()
     if (m_graphicsContext) {
         m_graphicsContext->resetCanvas(m_currentFrame->getRecordingCanvas());
         m_graphicsContext->setTrackOpaqueRegion(true);
+        m_graphicsContext->setTreatOverwriteAsOpaque(true);
     }
 }
 
@@ -45,6 +46,7 @@ void RecordingImageBufferSurface::setImageBuffer(ImageBuffer* imageBuffer)
     m_graphicsContext = imageBuffer ? imageBuffer->context() : 0;
     if (m_currentFrame && m_graphicsContext) {
         m_graphicsContext->setTrackOpaqueRegion(true);
+        m_graphicsContext->setTreatOverwriteAsOpaque(true);
         m_graphicsContext->resetCanvas(m_currentFrame->getRecordingCanvas());
     }
 }